The present invention relates to a combustor (burner) for waste gas treatment usable in combustion type waste gas treatment facilities for combustion-treating a harmful and combustible waste gas containing, for example, silane gas (SiH4) or a halogen gas (NF3, ClF3, SF6, CHF3, C2F6, CF4, etc.).
For example, a semiconductor manufacturing system discharges a gas containing harmful and combustible gases, e.g. silane (SiH4) and disilane (Si2H6). Such a waste gas cannot be emitted into the atmosphere as it is. Therefore, the common practice is to introduce such a waste gas into a pretreatment system where it is made harmless by oxidation through combustion. For this treatment, a method wherein flames are formed in a furnace by using an auxiliary combustible gas and the waste gas is burned with the flames is widely used.
Such a combustion type waste gas treatment system usually uses an auxiliary combustible gas consisting essentially of a fuel gas, e.g. hydrogen, city gas, or LPG, and an oxidizing agent, e.g. oxygen or air. The greater part of the running cost of the system is the cost for consumption of the fuel gas and the oxidizing agent. Accordingly, how much harmful waste gas can be destroyed efficiently with a minimum amount of auxiliary combustible gas is a measure of evaluating the performance of this type of system.
When silane, for example, is oxidized, silica (SiO2) is formed. Silica (SiO2) is a powdery substance, which may adhere to the wall surface of the combustion chamber and the burner ports, inducing poor combustion or causing clogging of the combustion chamber. Therefore, it is necessary to periodically carry out a cleaning operation to remove silica (SiO2). The cleaning is performed by a manual operation in the state of the art. Accordingly, the longer the cleaning operation interval, the easier the maintenance. The cleaning operation interval is also considered to be one of the important factors in evaluating the performance of a combustion type waste gas treatment system.
A general arrangement of a combustor used in a conventional combustion type waste gas treatment system as stated above is shown in FIGS. 23 and 24. A cylindrical combustion chamber 1 has a waste gas nozzle 2 provided in the center of the ceiling thereof to introduce a waste gas A to be treated into the combustion chamber 1. A plurality of auxiliary combustible gas nozzles 3 are provided around the outer periphery of the waste gas nozzle 2 to introduce an auxiliary combustible gas B into the combustion chamber 1. A combustion gas outlet 4 is integrally connected to the lower end of the combustion chamber 1. Thus, the waste gas A is passed through the center of flames formed in a side-by-side relation along a circle by the auxiliary combustible gas B injected from the auxiliary combustible gas nozzles 3. After it has passed, the waste gas A is mixed with the flames to burn. The resulting combustion gas is discharged to the outside from the combustion gas outlet 4.
At present, heat destruction is considered to be the most widely used method of destruction-treating a halogen gas, which is considered to be a main cause of global warming. That is, the destruction of a halogen gas needs high-temperature conditions created by a huge amount of heat or requires an enormous amount of excitation energy produced by plasma or the like. Using such a technique, destruction treatment of a halogen gas is carried out in destruction treatment equipment having a heating device, e.g. a heater, or a plasma generator and a complicated control mechanism, e.g. a safety device.
In the above-described conventional system, however, the flames of the auxiliary combustible gas are formed forward of the auxiliary combustible gas nozzles. Accordingly, the waste gas being injected forward from the waste gas nozzle, which is provided inside the auxiliary combustible gas nozzles, cannot always sufficiently mix with the flames of the auxiliary combustible gas. Therefore, the efficiency of destruction of the waste gas is not satisfactorily high. It is necessary, in order to increase the efficiency of destruction, to increase the amount of auxiliary combustible gas supplied so as to form large flames, thereby allowing the waste gas to burn easily. However, if the amount of auxiliary combustible gas supplied is increased, the amount of auxiliary combustible gas that does not contribute to the destruction of the waste gas also increases, causing an increase in running cost of the system. Moreover, silica (SiO2) resulting from the combustion of the waste gas undesirably adheres to the wall surface of the combustion chamber. Thus, depending on circumstances, the cleaning operation needs to be performed once or twice a week. In addition, destruction treatment of a halogen gas requires complicated equipment.
It should be noted that various techniques have been proposed to solve the above-described problems. For example, flames that are produced in side-by-side relation along a circle are so formed that the distal end of each flame slants toward the center of the circle, thereby allowing the waste gas to be efficiently exposed to the high-temperature portion of each individual flame. According to another technique, flame pipes are provided to maintain flames for an increased period of time, thereby allowing the flames and the waste gas to contact each other efficiently. However, it is deemed that these techniques have not completely solved the above-described problems. There has also been proposed a method of destruction-treating a halogen gas by using a combustor. With this method, however, the efficiency of destruction may vary to a considerable extent according to the rate of combustion. Therefore, the proposed method has not completely solved the above-described problems.
In view of the above-described circumstances, an object of the present invention is to provide a combustor for waste gas treatment usable in a combustion type waste gas treatment system, which provides a high efficiency of destruction of waste gas and yet allows the maintenance interval for cleaning to be lengthened and which is capable of destruction-treating a halogen gas with high efficiency.
According to a first aspect of the present invention, there is provided a combustor for waste gas treatment characterized by having a flame stabilizing zone surrounded by a peripheral wall and closed with a bottom wall. The flame stabilizing zone is provided to face a combustion chamber. A burner port for auxiliary combustible gas is provided in the peripheral wall to inject an auxiliary combustible gas into the flame stabilizing zone so as to produce a swirling flow. A burner port for waste gas is provided in the bottom wall to inject a waste gas into the flame stabilizing zone.
Thus, the auxiliary combustible gas is injected into the flame stabilizing zone so as to produce a swirling flow, thereby efficiently mixing the flame of the auxiliary combustible gas with the waste gas to be treated, and thus allowing the waste gas to be destroyed through combustion with high efficiency. Moreover, silica (SiO2) resulting from the combustion of silane gas or the like is prevented from adhering to the vicinities of the burner ports or to the wall surface of the combustion chamber by the swirling flame and the swirling flow. Thus, the waste gas can be stably treated through combustion for a long period of time.
In the case of a cylindrical combustion chamber, the peripheral wall can be formed by the inner peripheral surface of a cylindrical member.
It is preferable that a wall surface forming the combustion chamber be provided with an air injection nozzle for injecting air into the combustion chamber. Thus, the gas subjected to the combustion treatment is cooled with the air injected from the air injection nozzle. Moreover, the cooled combustion gas can be rapidly discharged out of the combustion chamber.
It is preferable that the air injection nozzle be provided so that air injected from the air injection nozzle forms a swirling flow in the combustion chamber. Thus, it is possible to cool the gas subjected to the combustion treatment, discharge the cooled gas out of the combustion chamber and remove silica (SiO2) from the wall surface of the combustion chamber even more effectively.
Further, it is preferable that the bottom wall be provided with a primary air injection nozzle for injecting primary air into the flame stabilizing zone. Thus, combustibility can be improved, and silica (SiO2) adhered to the surfaces of the inner and outer walls defining the flame stabilizing zone can be removed even more effectively.
It is preferable that the inner diameter of the combustion chamber and the inner diameter of the peripheral wall of the flame stabilizing zone be approximately identical with each other. Thus, a stagnant flow region is eliminated, and it is possible to prevent powdery silica (SiO2) from adhering to the inner wall of the flame stabilizing zone or the combustion chamber even more effectively.
An air nozzle for secondary combustion may be provided in the peripheral wall of the flame stabilizing zone downstream of the burner port for auxiliary combustible gas, whereby a reducing flame of primary combustion and an oxidizing flame of secondary combustion by the air are formed in the flame stabilizing zone, thereby making it possible to improve the efficiency of destruction of the waste gas, particularly a halogen gas.
According to a second aspect of the present invention, there is provided a combustor for waste gas treatment characterized by having a flame stabilizing zone surrounded by a peripheral wall and closed with a bottom wall. The flame stabilizing zone is provided to face a combustion chamber. A burner port for waste gas is provided in the bottom wall to inject a waste gas into the flame stabilizing zone. An air injection nozzle is provided in the peripheral wall of the flame stabilizing zone near the bottom wall to inject air so as to produce a swirling flow. A burner port for auxiliary combustible gas is provided in the peripheral wall of the flame stabilizing zone away from the bottom wall to inject an auxiliary combustible gas, such as a fuel gas or a premixed gas, into the flame stabilizing zone so as to produce a swirling flow.
Thus, an air flow is injected from the air injection nozzle provided in the peripheral wall of the flame stabilizing zone near the bottom wall so as to form a swirling flow. Therefore, the peripheral wall of the flame stabilizing zone can be cooled. Accordingly, the auxiliary combustible gas injecting from the burner port for auxiliary combustible gas, which is away from the bottom wall, is cooled, and thus stable combustion can be continued. Further, the swirling flow of flame is accelerated, so that it is possible to prevent silica (SiO2) resulting from the combustion of silane (SiH4) from adhering to the peripheral wall of the flame stabilizing zone or the combustion chamber even more effectively.
It is preferable that the auxiliary combustible gas be an over-rich premixed gas containing a fuel gas in excess of a stoichiometric amount. Thus, it is possible to form different flames, i.e. oxidizing flame and reducing flame, and hence possible to increase the efficiency of destruction of a halogen gas.
It is preferable that a secondary combustion air injection nozzle be provided in a wall surface extending from the peripheral wall of the flame stabilizing zone to form the combustion chamber or in a peripheral wall surface near the lower end of the flame stabilizing zone. Thus, the high-temperature region can be enlarged downward, and the efficiency of destruction of a halogen gas can be increased.
It is preferable that the inner diameter of the combustion chamber and the inner diameter of the peripheral wall of the flame stabilizing zone be approximately identical with each other. Thus, a stagnant flow region is eliminated, and it is possible to prevent powdery silica (SiO2) from adhering to the inner wall of the flame stabilizing zone or the combustion chamber even more effectively.
It is preferable that the burner port for auxiliary combustible gas be provided to face obliquely downward. Thus, it is possible to suppress heating of the cylindrical member and the rise in temperature and hence possible to extend the heat resistant life. In addition, it is possible to maintain the high-temperature state of the gas and to increase the efficiency of destruction of a halogen gas.